1. LECTURE #2 PRINCIPLES OF CONTROL SYSTEMS

2. Control systems integrate elements whose function is to maintain a process variable at a desired value or within a desired range of values Today's Objectives : DEFINE the following process control terms: a.Control system b.Control system input c.Control system output d.Open-loop system e.Closed-loop system f. Feedback g. Controlled variable h.Manipulated variable

3. Introduction Instrumentation provides the various indications used to operate a nuclear facility. In some cases, operators record these indications for use in day-to-day operation of the facility. The information recorded helps the operator evaluate the current condition of the system and take actions if the conditions are not as expected. Requiring the operator to take all of the required corrective actions is impractical, or sometimes impossible, especially if a large number of indications must be monitored. For this reason, most systems are controlled automatically once they are operating under normal conditions. Automatic controls greatly reduce the burden on the operator and make his or her job manageable. Process variables requiring control in a system include, but are not limited to, flow, level,temperature, and pressure. Some systems do not require all of their process variables to be controlled. Think of a central heating system.

4. A basic heating system operates on temperature and disregards the other atmospheric parameters of the hou se. The thermostat monitors the temperature of the house. When the temperature drops to the value selected by the occupants of the house, the system activates to raise the temperature of the house. When the temperature reaches the desired value, the system turns off. Automatic control systems neither replace nor relieve the op erator of the responsibility for maintaining the facility. The operation of the control systems is periodically checked to verify proper operation. If a control system fails, the operator must be able to take over and control the process manually. In most cases, understanding how the control system works aids the operator in determining if the system is operating properly a nd which actions are required to maintain the system in a safe condition.

5. Terminology A control system is a system of integrated elements whose function is to maintain a process variable at a desired value or within a desired range of values. The control system monitors a process variable or variables, then causes some action to occur to maintain the desired system parameter. In the example of the central heating unit, the system monitors the temperature of the house using a thermostat. When the temperature of the house drops to a preset value, the furnace turns on, providing a heat source. The temperature of the house increases until a switch in the thermostat causes the furnace to turn off. Two terms which help define a control system are input and output. Control system input is the stimulus applied to a control system from an external source to produce a specified response from the control system. In the case of the central heating unit, the control system inp ut is the temperature of the house as monitored by the thermostat

6. Control system output is the actual response obtained from a control system. In the example above, the temperature dropping to a preset value on the thermostat causes the furnace to turn on, providing heat to raise the temperature of the house. In the case of nuclear facilities, the input and output are defined by the purpose of the control system. A knowledge of the input and output of the control system enables the components of the system to be identified. A control system may have more than one input or output. Control systems are classified by the control action, which is the quantity responsibl e for activating the control system to produce the output. The two general classifications are open-loop and closed-loop control systems.

8. A closed-loop control system is one in which control action is dependent on the output. Figure 2 shows an example of a closed-loop control system. The control system maintains water level in a storage tank. The system performs this task by continuously sensing the level in the tank and adjusting a supply valve to add more or less water to the tank. The desired level is preset by an operator, who is not part of the system. Figure 2 Closed loop Control system

10. Automatic Control System An automatic control system is a preset closed loop control system that requires no operator action. This assumes the process remains in the normal range for the control system. An automatic control system has two process variables associated with it: a controlled variable and a manipulated variable. A controlled variable is the process variable that is maintained at a specified value or within a specified range. In the previous example, the storage tank level is the controlled variable. A manipulated variable is the process variable that is acted on by the control system to maintain the controlled variable at the specified value or within the specified range. In the previous example, the flow rate of the water supplied to the tank is the manipulated variable. Functions of Automatic Control In any automatic control system, the four basic functions that occur are: Measurement Comparison Computation Correction

11. Relationships between these elements and the functions they perform in an automatic control system are shown in Figure 4. The measuring element performs the measuring functi on by sensing and evaluating the controlled variable. The error detection element first compares the value of the controlled variable to the desired value, and then signals an error if a deviation exists between the actual and desired values. The final control element responds to the error signal by correcting the manipulated variable of the process. Figure 4 Relationships of Functions and Elements in an Automatic Control System

12. Feedback Control An automatic controller is an error-sensitive, self-correcting device. It takes a signal from the process and feeds it back into the process. Therefore, closed- loop control is referred to as feedback control.

14. CONTROL LOOP DIAGRAMS

15. A loop diagram is a "roadmap" that traces process fluids through the system and designates variables that can disrupt the balance of the system.

16. Terminology A block diagram is a pictorial representation of the cause and effect rel ationship between the input and output of a physical system. A block diagram provides a means to easily identify the functional relationships among the various components of a control system. The simplest form of a block diagram is the block and arrows diagram. It consists of a single block with one input and one output (Figure 5A). The block normally contains the name of the element (Figure 5B) or the symbol of a mathematical operation (Figure 5C) to be performed on the input to obtain the desired output. Arrows identify the direction of information or signal flow.

18. A takeoff point is used to allow a signal to be used by more than one block or summing point (Figure 7)

20. Below are several terms associated with the closed-loop block diagram. The plant is the system or process through which a particular quantity or condition is controlled. This is also called the controlled system. The control elements are components needed to generate the appropriate control signal applied to the plant. These elements are also called the "controller." The feedback elements are components needed to identify the functional relationship between the feedback signal and the controlled output. The reference point is an external signal applied to the summing point of the control system to cause the plant to produce a specified action. This signal represents the desired value of a controlled variable and is also called the "setpoint." The controlled output is the quantity or condition of the plant which is controlled. This signal represents the controlled variable. The feedback signal is a function of the output signal. It is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal. The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and feedback signal. This is also called the "error signal." The manipulated variable is the variable of the process acted upon to maintain the plant output (controlled variable) at the desired value. The disturbance is an undesirable input signal that upsets the value of the controlled output of the plant.

21. Figure 9 shows a typical application of a block diagram to identify the operation of a temperature control system for lubricating oil. (A) in Figure 9 shows a schematic diagram of the lube oil cooler and its associated temperature control system. Figure 9 Lube Oil Cooler Temperature Control System and Equivalent Block Diagram

22. oil reduces friction between moving mechanical parts and also removes heat from the components. As a result, the oil becomes hot. This heat is removed from the lube oil by a cooler to prevent both breakdown of the oil and damage to the mechanical components it serves. The lube oil cooler consists of a hollow shell with several tubes running through it. Cool ing water flows inside the shell of the cooler and around the outside of the tubes. Lube oil flows inside the tubes. The water and lube oil never make physical contact. As the water flows through the shell side of the cooler, it picks up heat from the lube oil through the tubes. This cools the lube oil and warms the cooling water as it leaves the cooler. The lube oil must be maintained within a specific operating band to ensure optimum equipment performance. This is accomplished by controlling the flow rate of the cooling water with a temperature control loop. The temperature control loop consists of a temperature transmitter, a temperature controller, and a temperature control valve. The diagonally crossed lines indicate that the control signals are air (pneumatic). The lube oil temperature is the controlled variable because it is maintained at a desired value (the setpoint)..

23. Cooling water flow rate is the manipulated variable because it is adjusted by the temperature control valve to maintain the lube oil temperature. The temperature transmitter senses the temperature of the lube oil as it leaves the cooler and sends an air signal th at is proportional to the temperature controller. Next, the temperature controller compares the actual temperature of the lube oil to the setpoint (the desired value). If a difference exists between the actual and desired temperatures, the controller will vary the control air signal to the temperature control valve. This causes it to move in the direction and by the amount needed to correct the difference. For example, if the actual temperature is greater than the setpoint value, the controller will vary the control air signal and cause the valve to move in the open direction. This results in more cooling water flowing through the cooler and lowers the temperature of the lube oil leaving the cooler. (B) in Figure 9 represents the lube oil temperature control loop in block diagram form. The lube oil cooler is the plant in this example, and its controlled output is the lube oil temperature. Thetemperature transmitter is the feedback element.It senses the controlled output and l ube oil temperature and produces the feedback signal. The feedback signal is sent to the summing point to be algebraically added to the reference input (the setpoint). Notice the setpoint signal is positive, and the feedback signal is negative. This means the resulting actuating signal is the difference between the setpoint and feedback signals

24. The actuating signal passes through the two control elements: the temperature controller and the temperature control valve. The temperature control valve responds by adjusting the manipulated variable (the cooling water flow rate). The lube oil temperature changes in response to the different water flow rate, and the control loop is complete. Process Time Lags In the last example, the control of the lube oil temperature may initially seem easy. Apparently, the operator need only measure the lube oil temperature, compare the actual temperature to the desired (setpoint), compute the amount of error (if any), and adjust the temperature control valve to correct the error accordingly. However, processes have the characteristic of delaying a nd retarding changes in the values of the process variables. This characteristic greatly increases the difficulty of control. Process time lags is the general term that describes these process delays and retardations. Process time lags are caused by three properties of the process. They are: capacitance, resistance, and transportation time. Capacitance is the ability of a process to store energy. In Figure 9, for example, the walls of the tubes in the lube oil cooler, the cooling water, and the lube oil can store heat energy. This energy- storing property gives the ability to retard change. If the cooling water flow rate is increased, it will take a period of time for more energy to be removed from the lube oil to reduce its temperature. Resistance is that part of the process that opposes the transfer of energy between capacities. In Figure 9, the walls of the lube oil cooler oppose the transfer of heat from the lube oil inside the tubes to the cooling water outside the tubes.

25. Transportation time is time required to carry a change in a process variable from one point to another in the process. If the temperature of the lube oil (Figure 9) is lowered by increasing the cooling water flow rate, some time will elapse before the lube oil travels from the lube oil cooler to the temperature transmitter. If the transmitter is moved farther from the lube oil cooler, the transportation time will increase. This time lag is not just a slowing down or retardation of a change; it is an actual time delay during which no change occurs. Stability of Automatic Control Systems All control modes previously described can return a process variable to a steady value following a disturbance. This characteristic is called "stability." Stability is the ability of a control loop to return a controlled variable to a steady, non- cyclic value, following a disturbance.

26. Control loops can be either stable or unstable. Instability is caused by a combination of process time lags discussed earlier (i.e., capacitance, resistance, and transport time) and inherent time lags within a control system. This results in slow response to changes in the controlled varia ble. Consequently, the controlled variable will continuously cycle around the setpoint value. Oscillations describes this cyclic characteristic. There are three types of oscillations that can occur in a control loop. They are decreasing amplitude, constant amplitude, and increasi ng amplitude. Each is shown in Figure 10. Decreasing amplitude (Figure 10A). These oscillations decrease in amplitude and eventually stop with a control system that opposes the change in the controlled variable. This is the condition desired in an automatic control system.

28. Constant amplitude (Figure 10B). Action of the controller sustains oscillations of the controlled variable. The controlled variable will never reach a stable condition; therefore, this condition is not desired. Increasing amplitude (Figure 10C). The control system not only sustains oscillations but also increases them. The control element has reached its full travel limits and causes the process to go out of control.

29. Control Loop Diagrams Summary A controlled system is the system or process through which a particular quantity or condition is controlled. Control elements are components needed to generate the appropriate control signal applied to the plant. These elements are also called the "controller." Feedback elements are components needed to identify the functional relationship between the feedback signal and the controlled output. Reference point is an external signal applied to the summing point of the control system to cause the plant to produce a specified action. Controlled output is the quantity or condition of the plant which is controlled. This signal represents the controlled variable. Feedback signal is a function of the output signal. It is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal. The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and feedback signal. This is also called the "error signal."

30. The manipulated variable is the variable of the process acted upon to maintain the plant output (controlled variable) at the desired value. A disturbance is an undesirable input signal that upsets the value of the controlled output of the plant. Process time lags are affected by capacitance, which is the ability of a processto store energy; resistance, the part of the process that opposes the transfer of energy between ca pacities; and transportation time, the time required to carry a change in a process variable from one point to another in the process. This time lag is not just a slowing down of a change, but rather the actual time delay during which no change occurs

32. TWO POSITION CONTROL SYSTEMS

33. A two position controller is the simplest type of controller. DESCRIBE the characteristics of the following types of automatic control systems: a. Two position control system

36. Two Position Controller A two position controller is a device that has two operating conditions: compl etely on or completely off. Figure 12 shows the input to output, characteristic waveform for a two position controller that switches from its "OFF" state to its "ON" state when the measured variable increases above the setpoint. Conversely, it switches from its "ON" state to its "OFF" state when t he measured variable decreases below the setpoint. This device provides an output determined by whether the error signal is above or below the setpoint. The magnitude of the error signal is above or below the setpoint. The magnitude of the error signal past that point is of no concern to the controller.

40. Deviation is the difference between the setpoint of a process variable and its actual value. This is a key term used when discussing various modes of control. Four modes of control commonly used for most applications are: proportional proportional plus reset (PI) proportional plus rate (PD) proportional plus reset plus rate (PID) Each mode of control has characteristic advantages and limitations. The modes of control are discussed in this and the next several sections of this module. In the proportional (throttling) mode, there is a continuous linear relation between value of the controlled variable and position of the final control element. In other words, amount of valve movement is proportional to amount of deviation. Figure 14 shows the relationship between valve position and controlled variable (temperature) characteristics of proportional mode. Notice that valve position changes in exact proportion to deviation. Also, the proportional mode responds only to amount of deviation and is insensitive to rate or duration of deviation. At the 2 minute and 4 minute marks, when the tempe rature returns to its setpoint value, the valve returns to its initial position. There is no valve correction without deviation.

44. RESET (INTEGRAL) CONTROL SYSTEMS

45. The output rate of change of an integral controller is dependent on the magnitude of the input. EO 1.4 DESCRIBE the characteristics of the following types of automatic control systems: c. Integral control

46. Reset Control (Integral) Integral control describes a controller in which the output rate of change is dependent o n the magnitude of the input. Specifically, a smaller amplitude input causes a slower rate of change of the output. This controller is called an integral controller because it approximates the mathematical function of integration. The integral control method is also known as reset control. Definition of Integral Control A device that performs the mathematical function of integration is called an integrator. The mathematical result of integration is called the integral. The integrator provides a linear outputwith a rate of change that is directly related to the amplitude of the step change input and a constant that specifies the function of integration. For the example shown in Figure 19, the step change has an amplitude of 10%, and the constant of the integrator causes the output to change 0.2% per second for each 1% of the input. The integrator acts to transform the step change into a gradually changing signal. As you can see, the input amplitude is repeated in the output every 5 seconds. As long as the input remains constant at 10%, the output will continue to ramp up every 5 seconds until the integrat or saturates

52. The controller acts to return the process to the setpoints. This is accomplishe d by the repositioning of the control valve. As the controller causes the control valve to reposition, the measured variable moves closer to the setpoint, and a new error signal is produced. The cycle repeats itself until no error exists. The integral controller responds to both the amplitude and the time duration of the error signal. Some error signals that are large or exist for a long period of time can cause the final control element to reach its "fully open" or "fully shut" position before the error is reduced to zero. If this occurs, the final control element remains at the extreme position, and the error must be reduced by other means in the actual operation of the process system